1. Technical Field of the Invention
This invention relates generally to audio amplifiers, and more specifically to amplifier biasing and volume control.
2. Background Art
Amplifiers are typically biased at a minimum bias level, in order to ensure good fidelity, low distortion, and so forth. In general, the most difficult case in which to achieve good fidelity is at the amplifier's maximum output level. Therefore, amplifier systems have traditionally had their bias set at a level such that the fidelity and performance are adequate at large signal operation.
At lower output level settings, this high bias level is typically far more than is necessary. This, unfortunately, results in an unnecessary and wasteful increase in power consumption when operating at small signal levels. This is especially unfortunate, given that most amplifiers are only rarely operated near their maximum output setting.
In general, it is desirable to minimize the power consumption of an amplifier circuit, particularly in battery-powered systems. It is also desirable to minimize the heat generated by the amplifier, to reduce costs associated with heatsinks, fans, and power supply components.
For a well-designed system which includes an output amplifier, the maximum output swing corresponds to the highest volume/level setting. Thus, the volume control setting and the output swing are related. In most systems, the volume control is implemented just before or as part of the power amplifier, or final stage in an amplifier chain, in order to maximize the dynamic range available to the preceding stages. In most systems, the maximum input level to the input of the volume control is known, and the peaks of the signal are generally made close to this limit, to maximize the signal-to-noise ratio of the preceding stages. Hence, it is possible to calculate the maximum possible output for each possible volume control setting.
FIG. 1 illustrates an exemplary audio system 10 according to the prior art. The audio system includes a multi-stage amplifier which includes a first stage (Amp Stage 1) which receives the audio input signal (often referred to as the “Voice Signal” regardless of its content), one or more intermediate amplifier stages (Amp Stage 2), and a final power amplifier stage (Amp Stage 3). The power amplifier is coupled to drive a headphone or loudspeaker transducer (LS). The power amplifier (or other stages) may include a feedback loop. The power amplifier has a power reference input coupled to the output of a bias generator. It is this bias generator whose bias level is set at a predetermined level, typically set to optimize fidelity and performance at the maximum possible output level of the power amplifier. The gain of the power amplifier is set by a volume control mechanism. Typically, the volume control mechanism is dynamically adjustable according to a setting established by a user.
What is needed is an improved audio amplifier system in which the gain established by the bias generator is dynamically adjustable, rather than being set at some predetermined point. Having a dynamically adjustable bias will enable the amplifier system to minimize power consumption and improve fidelity across a wide range of input signal levels and volume control settings.